Methods of synthesizing three-dimensional heteroatom-doped carbon nanotube macro materials and compositions thereof

ABSTRACT

Methods for synthesizing macroscale 3D heteroatom-doped carbon nanotube materials (such as boron doped carbon nanotube materials) and compositions thereof. Macroscopic quantities of three-dimensionally networked heteroatom-doped carbon nanotube materials are directly grown using an aerosol-assisted chemical vapor deposition method. The porous heteroatom-doped carbon nanotube material is created by doping of heteroatoms (such as boron) in the nanotube lattice during growth, which influences the creation of elbow joints and branching of nanotubes leading to the three dimensional super-structure. The super-hydrophobic heteroatom-doped carbon nanotube sponge is strongly oleophilic and an soak up large quantities of organic solvents and oil. The trapped oil can be burnt off and the heteroatom-doped carbon nanotube material can be used repeatedly as an oil removal scaffold. Optionally, the heteroatom-doped carbon nanotubes in the heteroatom-doped carbon nanotube materials can be welded to form one or more macroscale 3D carbon nanotubes.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to: provisional U.S. Patent ApplicationSer. No. 61/454,475, filed on Mar. 18, 2011, entitled “Methods ofSynthesizing Boron-Doped Carbon Nanotube Sponge-Like Materials andCompositions Thereof,” which provisional patent application is commonlyassigned to the assignee of the present invention and is herebyincorporated herein by reference in its entirety for all purposes.

GOVERNMENT INTEREST

This invention was made with government support under Grant NumberOISE-0756097 (titled “IRES-US-Mexico Collaborative Effort onNanotechnology Education and Research”) awarded by the National ScienceFoundation. This material is also based upon work supported by theNational Science Foundation Graduate Research Fellowship awarded to D.P. H. under Grant No. 0940902. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention relates to methods for synthesizingheteroatom-doped carbon nanotube sponge-like materials and compositionsthereof. More particularly, present invention relates to methods forsynthesizing three-dimensional (3D) carbon nanotube sponge-likematerials using heteroatom substitutional dopants and compositionsthereof. Such 3D carbon nanotube sponge-like materials includeboron-doped carbon nanotube sponge-like materials and sulfur-dopedcarbon nanotube sponge-like materials.

BACKGROUND OF THE INVENTION

Since the advent of carbon nanotubes (CNT) [Iijima 1991; Oberlin 1976]engineering and controlled synthesis of these materials have beenthoroughly investigated. Doping multi-walled carbon nanotubes (MWCNT)and single-wall carbon nanotubes (SWCNT) with elements such as nitrogenand boron has also been used for altering their electronic propertiesfor specific applications. [Stephan 1994; Suenaga 1997; Terrones 1996;Sen 1998; Nath 2000; Terrones 1999; Yudasaka 1997; Suenaga 2000; Wang2002; Kurt 2001; Ma 1999; Redlich 1996; Sen 1997]. Boron doping offullerenes by substitution was theoretically discussed [Han 2000]; andprevious research demonstrated that boron-doped multi-walled carbonnanotubes (CB×MWNTs) can be formed via chemical vapor deposition (CVD)using BF₃/MeOH as the boron source, a complex boron supported catalystFe/Ca (BO₃)₂/CaCO₃, and acetylene as the carbon source. [Mondal 2007] Itwas also found that “sea-cucumber” morphologies could be formed whencarrying out the spray-pyrolysis of a ferrocene-xylene-triethylborane(TEB) mixture. [Lozano-Castello 2004]. CB×MWNTs have been synthesizedusing 1M triethylborane in hexanes solution mixed with toluene andcompared to CN×MWNT's. [Koós 2010]. Other methods for synthesizingdouble-walled and single-walled CB×NT's have also been reported [Lyu2011; Goldberg 1999; Maultzsch 2002; Redlich 1996; McGuire 2005].

It has also been found that dopant atoms such as nitrogen or sulfur caninduce dramatic tubule morphology changes in CNTs, including covalentmulti-junctions [Sumpter 2007; Romo-Herrera 2008; Romo-Herrera 2009;Sumpter 2009], however these morphologies were not utilized to create 3Dmacro-scale architectures.

Theoretical and experimental studies on the electronic structure of bothsemiconducting and metallic CB×NT's have shown a strong acceptor statedue to the presence of boron and a lowering of the Fermi level. [Yi1993; Carroll 1998]. Theoretical studies have predicted that significantstructural reorganization generates stable bends in CNTs due to presenceof pentagon and heptagon defects [Dunlap 1992] that could accommodateforeign atoms besides carbon within the sp² graphitic lattice [Sumpter2009]. In addition, it was found that boron doping acts as a“surfactant” during growth to significantly increases the aspect ratioof nanotubes by preventing tube closure—allowing longer tube lengths tobe synthesized (˜5-100 μm) and favoring the zigzag (or near zigzag)chirality [Blasé 1999]. It was later found that dopant atoms can alsoinduce dramatic tubule morphology changes in CNT's. [Lee 2002; Sumpter2007].

CB×MWNTs could be synthesized by chemical vapor deposition (CVD) usingmultiple hydrocarbons and boron sources [Mondal 2007; Lozano-Castello2004; Koós 2010; Lyu 2011], but none of these works yielded macroscale3D solid structures, or were able to confirm the distinct tubularmorphologies induced by boron.

Theoretical and experimental research had demonstrated that boroninterstitial atoms located between double-walled CNTs act as atomic“fusers” or “welders” under high temperature annealing (1400-1600° C.)[Endo 2005], thus establishing covalent tube interconnections, butneither did this work produce macroscale solids. 3D solids of straightentangled non-doped CNTs were recently reported by others to createcompressible sponges [Gui II 2010] and temperature-invariantviscoelastic solids [Xu 2010]. However neither of these works showpromise towards any degree of covalent bonding established between CNTs;nor do they possess dramatic defect sites within the CNT network.

There is a need for a scalable synthesis process for building macroscalethree-dimensional structures from one-dimensional (1D) CNT buildingblocks. As used herein, a “macroscale three-dimensional structure” (or“macroscale 3D structure”) is a material that is at least 1 cm in threeorthogonal directions. The macroscale 3D structure composed of CNTs canbe obtained by: (1) randomly aligned, isotropic ensemble of entanglednanotubes without requiring nanotube-nanotube junctions; (2) randomlyaligned isotropic ensemble or an ordered array nanotube structurecontaining two-dimensional nanotube junctions; (3) randomly alignedisotropic ensemble or an ordered array nanotube structure containingthree-dimensional nanotube junctions; (4) a structure composed of anycombination of (1), (2) and (3). As further used herein, a “junction” isconsidered to be any form of covalent bonding between the nanotubes atany (all) angle(s).

As further used herein, a “two-dimensional nanotube junction” (which isalso referred to as a “two-dimensional nanotube” or “2D CNT”) is ananotube that is a least 100 nm in two perpendicular directions (or inthe same 2D plane having any angle), while, in the direction orthogonalto both perpendicular directions (2D plane), the nanotube is generallyless than 100 nm. For example, a nanotube in the shape of a cross (or an“X”) that has a length of more than 100 nm along the vertical axis and awidth of more than 100 nm along the vertical axis, but has a depth lessthan 100 nm, would be a two-dimensional nanotube junction. When atwo-dimensional nanotube junction is on the macroscale, this means thecarbon nanotube is at least 1 cm in two perpendicular directions, andcan be referred to as a “macroscale two-dimensional nanotube junction”(or a “macroscale two-dimensional nanotube.”)

Coordinately, a “three-dimensional nanotube junction” (which is alsoreferred to as a “three-dimensional nanotube” or “3D CNT”) is a nanotubethat is a least 100 nm in three orthogonal directions. When athree-dimensional nanotube junction is on the macroscale, this means thecarbon nanotube is at least 1 cm in three orthogonal directions, and canbe referred to as a “macroscale two-dimensional nanotube junction” (or a“macroscale three-dimensional nanotube.”)

In the present invention, the heteroatom-doped carbon nanotube material(such as CB×NT material) synthesized by the AACVD process of the presentinvention is a macroscale 3D structure (or macroscale 3D material). Theheteroatom-doped carbon nanotubes in this macroscale 3D structure arenot necessarily macroscale three-dimensional nanotubes. Generally, inthe absence of a post-synthesis process (such as welding) theheteroatom-doped carbon nanotubes in the macroscale 3D structure aretwo-dimensional nanotubes, and, in some instances, macroscaletwo-dimensional nanotubes, or purely entangled randomly alignedone-dimensional nanotubes.

After the post-synthesis process (such as welding), the nanotubes in themacroscale 3D structure can themselves be three-dimensional nanotubes,and, generally, macroscale three-dimensional nanotubes. In this way thestructure can become virtually monolithic solids composed of macroscalethree-dimensional nanotubes.

SUMMARY OF THE INVENTION

It has been discovered that heteroatom (such as boron, sulfur, nitrogen,phosphorous) doping can be used to build macroscale three-dimensionalstructures from CNTs in a scalable synthesis process, which results inthe formation of heteroatom-doped carbon nanotube materials, such asboron-doped carbon nanotube (“CB×NT”) materials. Herein, with respect todoping with heteroatoms, the term “doping” refers to placing heteroatomsin the CNT lattice in place of carbon atoms. A “heteroatom-doped CNT” isa carbon nanotube that has heteroatoms replacing carbon atoms in the CNTlattice.

An abundance of localized and topological defects, including extremetubular morphologies, are impactful features for many applicationsrequiring further CNT functionalization chemistry, or anchor-sites formolecular/atomic/nanoparticle adsorption (decoration) within the 3Dporous solid. Furthermore, substitutionally doped CNTs provide enhancedchemical reactivity. It is believed that the present invention is thefirst demonstration to exploit the uniqueness of heteroatomsubstitutional dopant effects on CNT morphology to create elasticmacroscale 3D structures. In combination with the heteroatom's (such asboron's) interstitial “welding” and “surfactant” effects, the dopingroute is directed to true (covalent) macroscale 3D carbon nanotubes,such as CNT monoliths, or interlocked nanotube ring structures proposedby Gogotsi [Gogotsi 2010], which had been studied theoretically asfascinating future materials with superior mechanical and electricalproperties [Romo-Herrera 2007]. The present invention includessubstitutional doping effects of boron (or other heteroatoms such assulfur, nitrogen, phosphorus, etc.) in carbon nanotubes so as to createa networked CB×NT solid (a macroscale 3D material). These materialspossess intriguing dynamic mechanical properties and can be used, forexample, as a reusable oil sorbent scaffold material in seawater.

Macroscopic quantities of three-dimensionally networked CB×NT materials(or other heteroatom-doped carbon nanotube materials) are directly grownusing an aerosol-assisted chemical vapor deposition method. The porousnanotube sponge is created by doping of boron (or other heteroatomsource) in the nanotube lattice during growth, which influences thecreation of elbow joints and branching of nanotubes leading to the threedimensional super-structure. The resulting materials have uniqueproperties. For instance, the super-hydrophobic CB×NT material isstrongly oleophilic and can soak up large quantities of organic solventsand oil(s). Due to this property, the CB×NT materials are sometimesreferred to as “CB×NT sponges” or “CB×NT sponge-like materials.” Thetrapped oil can be burnt off and the material can be used repeatedly asan oil removal scaffold.

The growth of macroscale (cm³ in size) 3D networked heteroatom-dopedcarbon nanotube materials (such as CB×NT materials) can be made directlythrough an efficient large-scale AACVD synthetic process. Detailedelemental analysis revealed the heteroatom (such as boron) to beresponsible for these results and creates “elbow-like” junctions andcovalent nanojunctions. These observations are in agreement with firstprinciple calculations—indicating that the most suitable sites to hostheteroatoms within a defective sp²-hybridized carbon network are closeto heptagonal rings or negatively curved areas. These macroscale 3Dheteroatom-doped carbon nanotube frameworks contain many functionaldefect-sites, which can be an advantage over pristine carbon nanotubecounterparts. To this end, heteroatom-doped carbon nanotubes have broadimplications for many practical material applications, such as selectivesorbent materials, hydrogen storage [Singh 1993; Froudakis 2011], andflexible conductive scaffolds as porous 3D electrodes.

The ultra-lightweight macroscale 3D material exhibited a variety ofmulti-functional properties including robust elastic mechanicalproperties with high damping, electrical conductivity, thermalstability, high porosity, super-hydrophobicity, oleophilic behavior andstrong ferromagnetism. The environmental oil removal-and-salvageapplication from seawater was demonstrated where the CB×NT sponge actsas an efficient scaffold which can be controlled and recollected via amagnetically driven process, and reused multiple times.

In general, in one aspect, the invention features a method of making amacroscale 3D heteroatom-doped carbon nanotube material. The methodincludes forming a chemical precursor solution comprising a carbonsource, a catalyst source, and a heteroatom source. The method furtherincludes generating an aerosol from the chemical precursor solution. Themethod further includes performing an aerosol-assisted chemical vapordeposition process using the aerosol to form the macroscale 3Dheteroatom-doped carbon nanotube material. The macroscale 3Dheteroatom-doped carbon nanotube material include heteroatom-dopedcarbon nanotubes.

Implementations of the inventions can include one or more of thefollowing features:

The heteroatom can be boron.

The heteroatom-doped carbon nanotubes can include two-dimensionalheteroatom-doped carbon nanotubes.

The carbon source can include toluene.

The carbon source can be toluene, cyclohexane, heptane, pentane,xylenes, hexanes, benzene, or a combination thereof.

The carbon source can include a liquid hydrocarbon that is capable ofdissolving the catalyst source, the heteroatom source, or both.

The carbon source can be at least 87 wt % of the carbon source, thecatalyst source, and the heteroatom source in the chemical precursorsolution.

The carbon source can be between about 87 wt % and about 97 wt % of thecarbon source, the catalyst source, and the heteroatom source in thechemical precursor solution.

The catalyst source can be capable of catalyzing the formation of carbonnanotubes in an aerosol-assisted chemical vapor deposition process.

The catalyst source can include a metal catalyst.

The metal catalyst can include iron, nickel, cobalt, an alloy thereof,or a combination thereof.

The metal can be iron.

The catalyst source can includes ferrocene.

The catalyst source can include a metallocene.

The metallocene can be ferrocene, nickelocene, cobaltocene, or acombination thereof.

The catalyst source can be between about 2.5 wt % and about 12 wt % ofthe carbon source, the catalyst source, and the heteroatom source in thechemical precursor solution.

The catalyst source can be between about 2.5 wt % and about 10 wt % ofthe carbon source, the catalyst source, and the heteroatom source in thechemical precursor solution.

The heteroatom source can include a boron source.

The boron source can include triethylborane.

The boron source can be an organoborane, an organoborate, or acombination thereof.

The boron source can be trimethylborane, triphenylborane,trimesitylborane, tributylborane, triethylborane, boric acid, trimethylborate, triisopropylborate, triethyl borate, triphenyl borate, tributylborate, diethylmethoxyborane, or a combination thereof.

The heteroatom can be boron, sulfur, nitrogen, phosphorus, or acombination thereof.

The heteroatom source can be a boron source, a sulfur source, a nitrogensource, a phosphorus source, or a combination thereof.

The heteroatom source can include a sulfur source.

The sulfur source can be amorphous sulfur powder, thiophene, allylsulfide, allyl methyl sulfide, dibenzothiophene, diphenyl disulfide, ora combination thereof.

The heteroatom source can be at most about 2 wt % of the carbon source,the catalyst source, and the heteroatom source in the chemical precursorsolution.

The heteroatom source can be between about 0.1 wt % and about 2 wt % ofthe carbon source, the catalyst source, and the heteroatom source in thechemical precursor solution.

The catalyst source can include metal atoms. The heteroatom source caninclude heteroatoms. The ratio of the metal atoms to the heteroatoms canbe between 2 and 20.

The ratio of the metal atoms to the heteroatoms can be between 4 and 6.

The carbon source can include toluene. The catalyst source can includeferrocene. The boron source can include triethylborane.

The carbon source can be between about 87 wt % and about 97 wt % of thecarbon source, the catalyst source, and the boron source in the chemicalprecursor solution. The catalyst source can be between about 2.5 wt %and about 12 wt % of the carbon source, the catalyst source, and theboron source in the chemical precursor solution. The boron source can bebetween about 0.1 wt % and about 2 wt % of the carbon source, thecatalyst source, and the boron source in the chemical precursorsolution.

The step of forming the chemical precursor solution can include (a)mixing the carbon source, catalyst source, and heteroatom source, and(b) sonicating the mixture of the carbon source, catalyst source andboron.

The aerosol can be introduced into a reactor capable of performing theaerosol-assisted chemical vapor deposition process using the aerosol toform the heteroatom-doped carbon nanotube material. The aerosol can beintroduced into the reactor via a carrier gas stream.

The carrier gas stream can include argon or argon/hydrogen balanced gas.

The carrier gas stream can be introduced into the reactor at a gas fluxrange between about 0.05 sl/min-cm² and about 0.6 L/min-cm².

The reactor can include a horizontal quartz hot-wall reactor chamber.

The aerosol-assisted chemical vapor deposition process can be carriedout under atmospheric pressure and at a temperature between 800° C. and900° C.

The can further include performing a functionization process tofunctionalize the heteroatom-doped carbon nanotubes in the macroscale 3Dheteroatom-doped carbon nanotube material.

The heteroatom-doped carbon nanotubes can be functioned withsubstituents that are metal particles.

The metal nanoparticles can be Au, Pt, Ag, Pd, Ti, Sc, Ni, V, or acombination thereof.

The heteroatom-doped carbon nanotubes can be functionalized withsubstituents that are chemical receptors, polymers, and/or proteins.

The method can further include forming a polymer composite that includesthe macroscale 3D heteroatom-doped carbon nanotube material.

The step of forming the polymer composite can include (a)functionalizing the heteroatom-doped carbon nanotubes with substituentsand binding the polymer to the functionalized heteroatom-doped carbonnanotubes, (b) directly binding polymer to the unfunctionalizedheteroatom-doped carbon nanotubes, or (c) embedding the heteroatom dopedcarbon nanotubes material in a matrix of the polymer.

The macroscale 3D heteroatom-doped carbon nanotubes can include one ormore macroscale 3D heteroatom-doped carbon nanotubes.

The method can further include welding the heteroatom-doped carbonnanotubes in the heteroatom-doped carbon nanotube material to form oneor more macroscale 3D heteroatom-doped carbon nanotubes in theheteroatom-doped carbon nanotube material.

The method can further include microwaving the heteroatom-doped carbonnanotube material to form one or more macroscale 3D heteroatom-dopedcarbon nanotubes in the heteroatom-doped carbon nanotube material.

In general, in another aspect, the invention features a macroscale 3Dheteroatom-doped carbon nanotube material that includes heteroatom-dopedcarbon nanotubes.

Implementations of the inventions can include one or more of thefollowing features:

The bulk density of the macroscale 3D heteroatom-doped carbon nanotubematerial can be between 10 mg/cm³ and 29 mg/cm³. The average diameter ofthe heteroatom-doped carbon nanotubes in the heteroatom-doped carbonnanotube material can be between 40 nm and 150 nm.

The heteroatom-doped carbon nanotube material can essentially beheteroatom-doped carbon nanotubes with little to no trace of amorphouscarbon.

The heteroatom-doped carbon nanotubes can have heteroatom induced elbowdefects.

The heteroatom-doped carbon nanotube material can have aweight-to-weight absorption capacity between about 22 and 123.

The macroscale 3D heteroatom-doped carbon nanotube material can becapable of absorbing a volume of solvent that is between about 70% andabout 115% of the volume of the macroscale 3D heteroatom-doped carbonnanotube material before absorption of the solvent.

The macroscale 3D heteroatom-doped carbon nanotube material can bemagnetic.

At least some of the macroscale 3D heteroatom-doped carbon nanotubes canbe functionalized macroscale 3D heteroatom-doped carbon nanotubes.

The composition can be a polymer composite including the macroscale 3Dheteroatom-doped carbon nanotube material.

The heteroatom can be boron, sulfur, nitrogen, phosphorus, or acombination thereof.

The macroscale 3D heteroatom-doped carbon nanotube material can includeat least one macroscale 3D heteroatom-doped carbon nanotube.

The macroscale 3D heteroatom-doped carbon nanotube material can be madeby the process including the steps of: (a) forming a chemical precursorsolution comprising a carbon source, a catalyst source, and a heteroatomsource; (b) generating an aerosol from the chemical precursor solution;and (c) performing an aerosol-assisted chemical vapor deposition processusing the aerosol to form the macroscale 3D heteroatom-doped carbonnanotube material.

In general, in another aspect, the invention features a method thatincludes selecting a macroscale 3D heteroatom-doped carbon nanotubematerial. The method further includes using the macroscale 3Dheteroatom-doped carbon nanotube material in an application that is: (i)an organic/oil cleanup process; (ii) a water purification process; (iii)an electrode material for a supercapacitor/battery device; (iv) anelectrode material for a battery device; (v) a scaffold support fortissue engineering and cell growth; (vi) a process to sense hazardousgasses at concentrations in the ppm range; (vii) a mechanical sensorapplication; (viii) hydrogen storage, or (ix) neutron radiationabsorption applications.

Implementations of the inventions can include one or more of thefollowing features:

The macroscale 3D heteroatom-doped carbon nanotube material can be usedin an organic/oil cleanup process.

The macroscale 3D heteroatom-doped carbon nanotube material can be usedto absorb organic/oil material in the organic/oil cleanup process.

The macroscale 3D heteroatom-doped carbon nanotube material theorganic/oil material can be mechanically removed from the macroscale 3Dheteroatom-doped carbon nanotubes. The macroscale 3D heteroatom-dopedcarbon nanotube materials can be reused to absorb further organic/oilmaterial.

The organic/oil material mechanically removed from the macroscale 3Dheteroatom-doped carbon nanotube materials can be salvaged.

The macroscale 3D heteroatom-doped carbon nanotube material theorganic/oil material can be burned out from the macroscale 3Dheteroatom-doped carbon nanotubes. The macroscale 3D heterodoped carbonnanotube materials can be reused to absorb further organic/oil material.

The heteroatom can be boron.

DESCRIPTION OF DRAWINGS

For a more detailed understanding of the preferred embodiments,reference is made to the accompanying figures, wherein:

FIG. 1A is a photographic image of macroscale 3D CB×NT material asproduced.

FIG. 1B is the macroscale 3D CB×NT material of FIG. 1A showing itsflexibility and mechanical stability upon being bent by hand.

FIG. 1C is an SEM image of an ion beam slice of the macroscale 3D CB×NTmaterial after ion beam slice and view feature showing that the interiorporous structure. The scale is 10 μm.

FIG. 1D is an SEM image showing a magnified view of the “elbow” defectsfound in CB×NTs of the CB×NT material. The scale is 200 μm.

FIG. 1E is an STEM image showing two, four-way covalent nanojunctions inseries of the CB×NT material. The scale is 200 μm.

FIG. 1F is a TEM image showing two overlapping CB×NT's (in the CB×NTmaterial) welded together assisted by boron doping. The scale is 10 μm.

FIG. 2A is a photograph of macroscale 3D CB×NT material taken undersunlight.

FIG. 2B is another photograph of macroscale 3D CB×NT material takenunder sunlight.

FIG. 2C is a photograph of macroscale 3D CB×NT material taken on thecontoured shape of a 1 inch diameter quartz tube in a reaction chamber.

FIG. 2D is another photograph of macroscale 3D CB×NT material taken onthe contoured shape of a 1 inch diameter quartz tube in a reactionchamber.

FIG. 2E is a photograph showing a water droplet 201 that beaded-up oncontact with the surface of the CB×NT material, which is indicative ofthe super-hydrophobicity of the CB×NT material.

FIG. 3A is an x-ray diffraction pattern of a sample of macroscale 3DCB×NT material,

FIG. 3B is an x-ray diffraction pattern of a sample of pristine(undoped) carbon nanotube material.

FIG. 4A is a graph showing the Raman spectroscopy comparison of pristinecarbon nanotubes (curve 401) with a CB×NT material (curve 402) using a514 nm wavelength laser.

FIG. 4B is a graph showing the Raman spectroscopy comparison of thepristine carbon nanotubes (curve 403) with a CB×NT material (curve 404)using a 633 nm wavelength laser.

FIGS. 5A-5H are high resolution transmission electron microscopy (HRTEM)images showing “elbow” defects of CB×NTs.

FIGS. 6A-6F are additional SEM images of the elbow nanojunctions(indicated by arrows) found in CB×MWNT's. Inset 601 of FIG. 6B shows acomputer generated model of different views of the pentagon-heptagonpair induced by the presence of boron in the nanotube lattice (note thechange in chirality of the tubes). Elbow defects occurred continuous andsomewhat at periodic distance intervals along the tube length of anindividual CB×NT.

FIG. 7A is a N₂ absorption isotherm that shows a type-II adsorptionisotherm that exhibits a negligible concave section, which is known tobe attributed to macroporous volume uptake, and a rapid rise in totalvolume near P/P₀=1 indicating a macroporous material. FIG. 7A shows theisotherm plots 701 and 702 for A and D, respectively.

FIG. 7B is the BET plots 703 and 704 for A and BF that shows a surfacearea of 103.24 m²/g for a high density sample.

FIG. 7C is the BET plots 705 and 705 for A and BF that shows a surfacearea of 360.44 m²/g for another high density sample.

FIG. 8 is the theoretical calculations using Density Functional Theory(DFT) within the GGA/PBE approximation defining stable dopant sites.Relative substitutional energies for B, N, and S dopant for variouspositions along a (5,5)/(9,0) nanotube knee (boomerang-type structure)as shown in lines 801-803, respectively. The short dashed lines 804-809correspond to substitutional energy in a straight (9,0) and (5,5) tube,respectively. The table in FIG. 8 shows the energy average over thesubstitutional sites located at the pentagonal and heptagonal kneeposition (the energy is relative to that in a periodic (5,5) nanotube).Boron does not promote any type of closure but rather strongly favorsstructure with a large number of regions exhibiting negative curvature.

FIGS. 9A-9G are high angle annular dark field (HAADF) images of theCB×NT elbow defects and their corresponding EELS line scans. FIG. 9A isa HAADF image showing the linescan path (arrows 901-902) along thenegative curvature of elbow defect profiles for elemental boron counts.The scale bar of FIG. 9A is 50 nm. FIG. 9B is a graph that shows theEELS lineprofile for line scans 901-902. The highest density of boronwas found at the onset of the negative curvature. FIG. 9C is an HAADFimage showing linescans performed on different locations (arrows903-905) mapping both elemental boron (B) and carbon (C) profiles. Thescale bar of FIG. 9C is 50 nm. FIG. 9D is a graph that shows the EELSlineprofiles for line scan 903 mapping B (profile 906) and C (profile907). FIG. 9E is a graph that shows the EELS lineprofiles for line scan904 mapping B (profile 908) and C (profile 909). FIG. 9F is a graph thatshows the EELS lineprofiles for line scan 905 mapping B (profile 910)and C (profile 911). FIGS. 9D-9F appear to follow the same trend asfurther evidence of boron incorporation into the lattice. FIG. 9F showsthe boron appears to be more easily detected at the outer layers nearthe regions of high negative curvature. FIG. 9G is a graph of thebackground subtracted EELS spectrum of the CB×NT's showing both the Cand B characteristic K-shell peaks.

FIGS. 10A-10E are SEM images showing EELS linescans 1001-1005,respectively performed along the positive (arrows 1006) and negative(arrows 1007) curvature regions on the “elbow” defect side. FIG. 10F isa graph that shows the corresponding B:C ratio from the EELS linescans1001-1005 (of FIGS. 10A-10E, respectively). FIG. 10G is the portion ofFIG. 10F that corresponds to linescale 1003. Arrows 1008 point to thepeaks near the negative curvature regions surrounding the positivecurvature regions along the “elbow” bend. The circles 1009 indicatewhere the linescans extend into the “vacuum region” representingartifacts from the measurement.

FIG. 11 is a graph showing XPS characterizing boron bonding states andcontent. Curve 1101 is the raw data signal. Curves 1102-1106 aredeconvoluted B1s bonding states, after argon ion-etch, at peak positions187.6, 188.8, 190.1, 191.8, and 193.4 eV, respectively, corresponding tothe B—C bonding in B₄C (187.8 eV), B-substituted-C within the hexagonallattice (188.8 eV), BC₂O (190.0 eV), BCO₂ (192.0 eV), and B₂O₃ (193.2eV). The majority bonding state is B-substituted-C and BC₂O on theCB×MWNT surface, and the boron content is ca. 0.7 at %. Curve 1107 isthe peak fitting curve.

FIG. 12A is a graph of stress-strain curves 1201-1204 at 60% at 0.5 Hzand room temperature for cycles of 1, 2, 50, and 100, respectively, on asample of CB×NT material with density ˜27 mg/cm3.

FIG. 12B is a computer graphics model that shows the entangled randomnetwork as compared to conventional CNT arrays. The elbow defects aid inrecovery via spring-back loading mode, shown in steps 1205-1207, toovercome the van der Waal ‘sticking’ force of contacting tubes uponcompression which give rise energy dissipation and high tan δ values.

FIG. 12C is a graph showing the results of a dynamic mechanical analysis(DMA) on a CB×NT material (with density≈25 mg/cm³) using multi-strainmode at 1 Hz for 250 cycles. Curves 1208-1210 correspond to thecompressive strength, stiffness, and tan δ, respectively.

FIG. 12D is a graph showing the results of a dynamic mechanical analysis(DMA) on a CB×NT material (with density≈20 mg/cm³) using multi-strainmode at 1 Hz for 11,000 cycles. Curves 1211-1213 correspond to thecompressive strength, stiffness, and tan δ, respectively.

FIG. 13A is a photograph of superhydrophobic surface with a 150° contactangle measurement of a 2 mm diameter water droplet resting on the spongesurface. FIG. 13B is a contact angle measurement of a 2 mm diameterwater droplet on the surface of a CB×NT material using a Goniometerdevice.

FIG. 13C is a graph of the weight-to-weight absorption capacity definedby the ratio of (a) the final weight after solvent absorption to (b) theinitial weight of the sponge before absorption for common solvents, asmeasured on CB×NT material samples having different densities. Lines1301-1303 are for CB×NT material samples having densities of 24.3mg/cm³, 17.3 mg/cm³, and 10.8 mg/cm³, respectively.

FIGS. 13D-13G are photographs of demonstrations using CB×NT materials toclean up used engine oil spill (0.26 mL) in seawater; sample (m≈4.8 mg,ρ≈17 mg/cm³). FIG. 13D is a photograph that shows the material 1304dropped into the oil at t=0 min. Inset 1305 shows the material beforeuse. FIG. 13E is a photograph of material 1304 absorbing the oil at t=2min with inset 1306 showing material 1304 at t=5 min. FIG. 13F is aphotograph that shows by burning or squeezing (inset 1307) the oil canbe salvaged from material 1304. FIG. 13G is a photograph showing thatmaterial 1304 can then be reused repetitively. By using a magnet 1308,material 1304 can track or move the oil. Inset 1309 shows material 1304after burning and before reuse.

FIG. 14A is a graph of magnetization (emu/g) versus applied magneticfield (Oe) of a sample of CB×NT material.

FIG. 14B is a graph of the PPMS four-probe electrical resistivitymeasurement vs. temperature on bulk CB×NT material.

DETAILED DESCRIPTION

The present invention is a new method for synthesizing heteroatom-dopedcarbon nanotube materials, such as CB×NT materials. Otherheteroatom-doped carbon materials include sulfur doped carbon nanotubematerials.

In the present specification, boron-doped carbon materials are primarilydiscussed throughout the detailed description; however, this isrepresentative of doping carbon nanotubes with other heteroatoms.

Large-scalable CVD synthesis of CNTs (such as AACVD synthesis) with aheteroatom (such as boron) containing precursor resulted in “elbow”tubule morphologies forming sponge-like macroscale 3D materials ofentangled carbon nanotube networks. It is believed that the heteroatom(i.e., boron) was responsible for the formation of these “elbow”defects, which evidences structural morphology effects of substitutionaldoping with foreign atoms in the pristine carbon nanotube lattice. Theresulting heteroatom-doped carbon nanotube material (such as CB×NTmaterial) exhibited robust elastic mechanical properties, highelectrical conductivity, high porosity, super-hydrophobicity, oleophilicbehavior, and strong magnetism. The combination of these propertiesenable this novel macroscale 3D structure of nanotubes for use invarious applications, including in environmental organics/oil cleanupand water purification technologies.

The present invention entails a newly specified precursor formula andexperimental parameters/processing conditions on an existingtechnological materials synthesis method (chemical vapor deposition) tocreate an entirely new form of carbon nanotube material. Morespecifically, this new form is heteroatom doped carbon nanotubematerial, such as boron-doped carbon nanotube (CB×NT) materials, whichare macroscale 3D materials.

In embodiments of the present invention, the invention is a compositionof matter to be synthesized via an aerosol assisted CVD technique.Embodiments of the invention include:

-   -   (a) Forming of the heteroatom-doped carbon nanotube material;    -   (b) Characterization of the heteroatom doped carbon nanotube        material;    -   (c) Functionalization of the heteroatom-doped carbon nanotube        material;    -   (d) Polymer composites of the heteroatom-doped carbon nanotube        material; and    -   (e) Use of the heteroatom doped carbon nanotube material in        processes.        Synthesis of the Heteroatom Doped Carbon Nanotube Material

Synthesis Process

The heteroatom-doped carbon nanotube synthesis process takes advantageof the doping effect of heteroatoms (such as boron) on tubule morphologyin order to create the three-dimensional entangled networkedheteroatom-doped carbon nanotube materials (such as macroscale 3D CB×NTmaterials).

In an embodiment of the invention, CB×NT material (multi-walled carbonnanotubes) was grown directly on the walls of a quartz tube furnace viaa chemical vapor deposition (CVD) method, and more specifically anaerosol-assisted chemical vapor deposition (AACVD), using triethylborane(TEB) (Aldrich >95%) as the boron source.

The AACVD process can be carried out under atmospheric pressureconditions and can include a horizontal quartz hot-wall reactor chamberheated by a tube furnace in the temperature range of 800-900° C. Theprocess involves the use of chemical precursor solutions that include acarbon source, a catalyst source, and a heteroatom source (such as aboron source).

The carbon source is generally an aromatic hydrocarbon chemical liquidhydrocarbon usually in liquid form, such as toluene (C₇H₈) orcyclohexane (C₆H₁₂). Other carbon sources include heptane (C₇H₁₆),pentane (C₅H₁₂), xylenes (C₈H₁₀), hexanes (C₆H₁₄), and benzene (C₆H₆).Toluene is a good carbon source to utilize as it is also a solvent inwhich the other components of the chemical precursor solution can bedissolved. Generally, the carbon source is above 87% of the total weightof the carbon source, the catalyst source, and the heteroatom source inthe chemical precursor solutions. In some embodiments, the chemicalprecursor solutions can be prepared using between about 92 wt % andabout 97 wt % of toluene as the carbon source.

The catalyst source is generally a metal catalyst source, such as ametallocene in solid powder form. Typically, the metal catalyst sourceis an iron metal catalyst source, such as ferrocene (C₁₀H₁₀Fe). Othermetal catalyst sources include nickel metal catalyst sources, such asnickelocene (C₁₀H₁₀Ni), and cobalt metal catalyst sources, such ascobaltocene (C₁₀H₁₀Co), and combinations/alloys thereof.

In embodiments utilizing a metallocene, the metallocene (solid powder)concentration dissolved in the hydrocarbon (liquid) is generally between10 to 150 mg/mL. For instance, ferrocene (solid) concentration dissolvedin the toluene (liquid) is generally between 10 to 150 mg/mL.

Generally, the catalyst source is between 2.5 and 12 wt % of the totalweight of the carbon source, the catalyst source, and the heteroatomsource in the chemical precursor solutions. In some embodiments, thechemical precursor solutions can be prepared using between about 2.5 andabout 10 wt % of ferrocene as the catalyst source.

The heteroatom source is generally a liquid source of the heteroatom ora source of the heteroatom that will dissolve in the chemical precursorsolution. For example, when the heteroatom is boron, the boron source isa organoborane or organoborate chemical. For instance, the boron sourcecan be triethylborane (Aldrich >95%) (TEB) (C₆H₁₄B). Other organoboranesinclude trimethylborane (liquid) (C₆H₁₄B), triphenylborane (solid)(C₁₈H₁₅B), trimesitylborane (solid) (C₂₇H₃₃B), tributylborane (liquid)(C₁₂H₂₇B), and triethylborane. Organoborates can include boric acid, trimethyl borate, triisopropylborate, triethyl borate, triphenyl borate,tributyl borate, and diethylmethoxyborane. Boron trichloride (BCl₃) gascan also be used as a boron source (and mixed with the carrier gas).

Also, for example, when the heteroatom is sulfur, the sulfur source issulfur containing organic compound. The sulfur source can be pureamorphous sulfur powder or sulfur containing organic compound such asthiophene, allyl sulfide, allyl methyl sulfide, dibenzothiophene, ordiphenyl disulfide.

Generally, the heteroatom source is less than about 2 wt % of the totalweight of the carbon source, the catalyst source, and the heteroatomsource in the chemical precursor solutions. In some embodiments, thechemical precursor solutions can be prepared using between about 0.1 andabout 1.0 wt % of triethylborane (Aldrich >95%) (TEB) as the boronsource.

In some embodiments of the present invention, the chemical precursorsolutions were prepared using 87-96.9 wt. % toluene as the carbonsource, 2.5-12 wt. % ferrocene as the iron metal catalyst source andconcentrations varying between 0.1-2.0 wt. % triethylborane(Aldrich >95%) (TEB) as the boron source. These concentrations of thecarbon source, a catalyst source, and the boron source in the chemicalprecursor solutions can be varied depending on the desired properties ofthe material, such as density, porosity, surface area, carbon nanotubediameter, boron doping concentration, etc.

In some embodiments of the invention, the Fe:B, Ni:B, or Co:B (Fe:S,Ni:S, or Co:S) molar ratio within the solution (or gas mixture) isbetween 2 to 20, and typically between 4 and 6.

After mixing the carbon source, the catalyst source, and the boron (orother heteroatom) source together, this mixture can optionally besonicated, such as to speed up the dissolution of the catalyst sourceand/or the boron source in the chemical precursor solution. Thesonication can occur between about 15 minutes and an hour. Typically,the sonication occurs for around 30 minutes or more.

After preparation, the chemical precursor solution is placed in anaerosol generator to generate an aerosol, (i.e., micro-droplet (<10micron diameter) size mist cloud). For instance, an ultrasonic generatorcan be used to produce an ultrasonic beam directed at the surface of thechemical precursor solution, which forms the aerosol. Such aerosol canbe then transported to the reactor by flow of a carrier gas, such asargon (or other non-reactive gas). Examples of such ultrasonic aerosolgenerators include the Pyrosol 7901 type manufactures by RBIInstrumentation. The Pyrosol 7901 type generator is a vessel with anultrasonic piezoelectric transducer film at the bottom, controlled by anexternal generator with adjustable frequency and amplitude. During thisaerosol generation process, the aerosol is generated above the solution.

Other types of aerosol generators include ones that are injectionsystems similar to those utilized in the automobile industry. Thechemical precursor solution is stored in a tank, and then pushed under apressure (typically around 1 bar) by a carrier gas, such as argon, to avalve working in a pulsed mode.

After generation, the aerosol is then transferred into the reactorchamber using the carrier gas, such as argon. In some embodiments of thepresent invention, the carrier gas is introduced into the reactor at agas flux range between about 0.05 standard liters per minute per squarecentimeter (sl/min-cm²) and about 0.6 sl/min-cm², and typically betweenabout 0.20 sl/min-cm² and about 0.30 ml/min-cm². Thus range of fluxvalues can be used to determine the carrier gas feed rate that scalesinto the CVD system. For instance, when the gas flow of the carrier gasis through a 4.6 cm inner diameter tube (such as a 4.6 cm inner diameterquartz tube), a carrier gas flux of 0.24 sl/min-cm² would yield asolution feed rate of 4.0 sl/min-cm². Again, the carrier gas istypically argon. In some embodiments, the carrier gas can be anargon-hydrogen gas mixture.

Referring to the precursor solution in carrier gas, the precursorsolution can be introduced into the reactor at a gas flux range betweenabout 0.01 ml/min-cm² and about 0.5 ml/min-cm², and typically betweenabout 0.09 ml/min-cm² and about 0.15 ml/min-cm². Again, the range offlux values can be used to determine the solution feed rate that scalesinto the CVD system. For instance, when the gas flow of the carrier gasis through a 4.6 cm inner diameter tube (such as a 4.6 cm inner diameterquartz tube), a solution flux of 0.09 ml/min-cm² would yield a solutionfeed rate of 1.50 ml/min.

In the hot chamber reactor zone, the chemical precursor solution isevaporated and the heteroatom-doped carbon nanotube material (such asCB×NT material) is either prepared and collected on the wall of thereactor or is deposited and grown on a substrate. Typically, theheteroatom-doped carbon nanotube growth occurs on quartz/silicasubstrate in a quartz tube furnace.

An advantage of using an AACVD method is that the chemical precursorsolutions can be continuously feed into the reactor chamber, thusrendering the process commercially scalable.

For example, a three-dimensional (3D) bulk CB×NT material consistingentirely of CB×NTs was synthesized as follows:

The aerosol-assisted chemical vapour deposition (AACVD) system wascarried out under atmospheric pressure conditions and comprises ahorizontal hot-wall quartz tube reactor chamber heated by a furnace (30cm heating zone). Solutions were prepared mixing toluene (Aldrich,anhydrous, 99.8%) and ferrocene (Fe(C₅H₅)₂) (Alpha Aecer 99%) at aconcentration of 25 mg/mL, and triethylborane (TEB) ((C₂H₅)₃B)(Aldrich >95%) at Fe:B ratio 5:1, followed by 30 minute sonication. TheTEB was added while in a glove box under an inert nitrogen atmosphere.

The chemical precursor solution was placed in a glass vessel with anultrasonic piezoelectric transducer film (diameter=40 mm) at the bottom(Pyrosol 7901 type). The piezoelectric frequency and amplitude wascontrolled by an external generator source providing a resonantfrequency ˜0.8 MHz.

The chemical precursor solution feed rate was varied between 0.4-0.8ml/min for a total synthesis time of 30 minutes. The aerosol generatedabove the solution was transferred into the reactor chamber by an argon,or argon/hydrogen balanced, carrier gas (argon/hydrogen balanced gas ispreferred) at flow rates of 2.00-2.50 L/min. The furnace temperatureranged from 850° C.-870° C. in the chamber reactor zone where thechemical precursor solution was evaporated. The temperature of thefurnace may range from 800 to 900° C., but is usually between 840 to870° C. and more usually between 850° C. and 860° C.

Deposition and growth occurred directly onto the 1 inch diameter quartztube walls taking on the shape of the tube. The result producedquantities between 2 to 3 grams of CB×NT material in just 30 minutes ofgrowth (60-100 mg/min.), in the form of macroscopic elastic solids (seeFIGS. 1A-1F and FIGS. 2A-2E), exhibiting unique physico-chemicalproperties including oleophilicity. As boron can act as a surfactantduring growth [Blasé 1999], it is believed this could be a reason forthe high yield. The macroscale 3D CB×NT material could be bent to adramatic degree without breaking and returned to its original positionafter released. FIGS. 1A-1B. The CB×NT material had a robust mechanicaldurability and flexibility in response to ‘flicking’ the material byhand in a cantilever loading fashion. Remarkably, the bulk densities ofthe porous solids were measured to be in the range of 10 to 29 mg/cm³(as compared to low density carbon aerogel of 60 mg/cm³). Densitiesbelow 10 mg/cm³ may also be achieved by changing the solution feed rateand synthesis temperature accordingly. The nanotube diameters in theCB×NT material ranged from 40 to 150 nm, as measured from electronmicroscopy images. FIGS. 1C-1F. Diameters may be below 40 nm or evenbelow 20 nm by changing the synthesis parameters such as precursor feedrate, catalyst concentration, temperature, and carrier gas flow rate.The synthesized 3D architecture of the CB×NT material was entirely madeup of randomly orientated and entangled CNTs with little to no amorphouscarbon as depicted from SEM. Sec FIG. 1C.

As shown in FIGS. 3A-3B, the x-ray diffraction pattern showed that theas-produced CB×NT materials were crystalline and had sharp (002)diffraction peaks.

The x-ray diffraction pattern of CB×NT material (curve 301) as comparedto the x-ray diffraction pattern of pristine (undoped) carbon nanotubes(curve 302) showed evidence of peak broadening and a shift to lowerdiffraction angle of the (002) planes appeared for CB×NT material. Thisindicated an increase in the interplane d-spacing (Δd≈0.007 nm) betweenthe graphitic carbon nanotubes walls due to the boron substitutionallydoped with carbon creating disorder in the lattice.

At longer growth times and the lower solution feed rates, sponge-likematerials had lower density, more robust mechanical properties(toughness), higher porosity, and higher specific surface area, whilemaintaining very high electrical conductivity.

The catalytic role of boron (or other heteroatom) to prevent tubeclosure [Blasé 1999] was responsible to promoting extraordinarily highyield and efficient growth kinetics for doped carbon nanotubeproduction. It was found that the TEB content in the precursor had adirect relationship with the growth temperature needed for yielding thesolid structure. The successful growth conditions for the materials ofthe present invention were very sensitive to the TEB concentration.During growth optimization, it was noticed that the presence of TEBresulted in an increase in the reaction temperature. This observationmay be explained by the heteroatoms (such as boron atoms) starting tostrongly react with the iron catalyst particles to a degree that mayalter the carbon diffusion, saturation, and precipitation growthkinetics of long “elbow-defected” heteroatom-doped carbon nanotubes. Forthe CB×NT material, it was found that the Fe to B ratio ranges from 2 to6 within the temperature range 900 to 850° C. respectively. Therefore,the possible role of the catalytic effects of atomic boron on the ironcatalyst particles during CB×NT can be used to control nanotube 3Darchitectures. Using boron as a dopant in carbon nanotube synthesis is astrategy for producing “elbows,” which contribute to the elasticity ofthese networks. The structural integrity of the 3D heteroatom-dopedcarbon nanotube material is maintained due to the heteroatom induceddefects—promoting tube-tube bonding, entanglement, and nanoscalecovalent multi-junctions. See FIGS. 1B-1E. In this respect, the dopingroute seems to be more advantageous, over non-doped CNT entanglednetworks, holding more promise as a strategy for true (covalent) 3Dsolid networks with CNTs.

Post-Synthesis Welding Process

Optionally, the synthesized heteroatom-doped carbon nanotube materialmay be welded after the synthesis process. Accordingly, the inventioncan further entail a post-synthesis procedure to weld theheteroatom-doped carbon nanotube macroscale 3D material, such as byusing microwave radiation energy, for the purpose of enhancing materialproperties (mechanical, electrical, chemical reactivity).

The post-processing welding procedure enhances the degree of covalentjunctions between individual carbon nanotubes. This, in effect canenhance the overall material properties of the macroscale 3D MWCNTstructure.

In the synthesis process of the current invention, no substrate isneeded to provide a 3D distribution of nanotubes in space, such asdescribed in the Chen '258 Patent. The present invention provides amass-production method of forming the ideal framework of freestanding,randomly orientated, entangled MWCNTs distributed in 3D macro-scalespace. Simply drop casting a solution of carbon nanotubes (such asMWCNTs) onto a substrate (in a “pick-up-sticks” fashion) will yield aloose 2D distribution of MWCNTs, in which case, bundling up of CNTs dueto van der Waal forces is very difficult to avoid. In the presentinvention, bundling of the MWCNTs is avoided due to the “elbow” defectsand tube morphologies (bends, kinks, Y-, T-, and X-type junctions)induced by the heteroatom doping (boron. sulfur, etc.) which helps topromote the entanglement and to prevent the strong domination of van derWaal forces commonly known with conventional SWCNT and MWCNT randomlyorientated powders and anisotropic aligned arrays. The macroscale 3Dentangled network of MWCNTs that compose the heteroatom-doped carbonnanotube materials of the present invention, are therefore in more ideal3D fixed positions for contacting MWCNTs to weld together within thesolid to form macroscale 3D carbon nanotubes. This will result in avirtually monolithic network of carbon nanotubes (such as MWCNTs), whichwill enhance the overall material properties and performance (inparticular the mechanical and electrical properties) of these carbonnanotube elastic solids.

This present invention entails a welding post-processing procedure toprovide large-scale synthesis of interconnected carbon nanotube 3Dnetworks in the form of macroscopic solids (i.e., macro-scale 3Dmaterials) having further enhanced material properties and performance.Accordingly, the present invention entails the post-synthesis methodperformed on the aforementioned CVD synthesized structure for preparinginterconnected MWCNT networks in three-dimensional (3D) space to formmacro-scaled, porous, elastic solids with enhanced material properties.

This can be done by microwave irradiation welding technique to promotecrosslinking and create a virtually monolithic covalently bonded networkof interconnected carbon nanotubes and/or heteroatom-doped carbonnanotubes (boron, sulfur, nitrogen, or phosphorous). This can be doneusing the microwave energy parameters similar to those outlined in theHarutyunyan '884 Patent and the Tour '199 Patent as described forapplication strictly on pristine (non heteroatom-doped) SWCNT and MWCNTloose powders. These methods were for small-scale 2D layering of CNTs(2D stacking or packing of CNTs), which are vulnerable to the strong vander Waal forces rendering the process counterproductive and lessefficient to building true 3D porous solid network structures at themacro-scale. These similar parameters may be applied on the presentinvention; however in this case, the invention regards the applicationto 3D heteroatom-doped carbon nanotube materials.

The microwave radiation energy can come from a conventional microwaveoven, such as those used as a household appliance; in which case themicrowave frequency would be 2.45 GHz and powers that range from 600 to1400 watts. It is also possible to use other non-conventional microwavefrequencies between 1 to 300 GHz, and generally between 1 and 5 GHz.

The power output of the microwave radiation may also vary between 400watts and 1400 watts. Typically, conventional microwave radiationfrequency 2.45 GHz and power output between 600 and 1400 watts isutilized.

By this “welding process,” temperatures between 1000 and 2000° C. can bereached. Preferably temperatures above 1500° C. may be needed for thebreakdown of the carbon-carbon bonds and the reconstruction (welding) ofsp² crystalline covalent junctions (crosslinking) between individualcarbon nanotubes (such as MWCNTs).

Generally, the process is performed under inert atmosphere conditions,such as nitrogen or argon, to prevent significant oxidation or burningof the carbon nanotubes (such as MWCNTs) at elevated temperatures. Also,the material can be put under vacuum environment conditions such asthose below <1 torr, and more typically between 10⁻³ to 10⁻⁷ torr (orwithin an ultra high vacuum (UHV) chamber). The samples may be sealedwithin a quartz vessel under such pressure conditions as well.

In embodiments of the present invention, the heteroatom-doped carbonnanotubes can be chemically functionalized with functional groups beforethe microwave irradiated procedure.

Moreover, composites thereof may be constructed by such means. Forexample, embodiments of the present invention, can utilizeheteroatom-doped carbon nanotubes (functionalized or unfunctionalized)in combination with one or more of (a) carbon nanotubes doped with thesame heteroatom but functionalized with a different substituent, (b)carbon nanotubes doped with other heteroatoms (unfunctionalized orfunctionalized with the same or different substituent), (c) undopedcarbon nanotubes (unfunctionalized or functionalized with the same ordifferent substituent), (d) enhanced heteroatom (such as boron, sulfuretc.) atomic percentage/concentration within the CNT framework byinfiltrating additional dopant sources to react and increase/modify theheteroatom-doped carbon nanotubes etc.

By such welding process, the carbon nanotubes in the heteroatom-dopedcarbon nanotubes are covalently bonded resulted in 3D carbon nanotubes,and generally, macroscale 3D carbon nanotubes.

Characterization of the Heteroatom Doped Carbon Nanotube Material

The process of the present invention yielded gram quantities ofheteroatom-doped carbon nanotube material (such as CB×NT material) inthe form of macroscopic elastic solids exhibiting fascinatingphysico-chemical properties including oleophilicity (which can be, forexample, used as an efficient oil and solvent removal). For purposed ofthe present invention, CB×NT materials were characterized.

Remarkably, the bulk densities of the CB×NT material was measured to bein the range of 10-29 mg/cm³ (compared to low density carbon aerogel of60 mg/cm³) and may be prepared with less than 10 mg/cm³. The morphologyand structural properties of the CB×NT material was studied by SEM(FEI-field emission SEM-XL30 operated at 1-15 keV) and TEM/STEM (JEOL2010 F instrument equipped with a Gatan Enfina energy-lossspectrometer). As measured from SEM images, it was found that thenanotube diameters of the CB×NTs in the CB×NT material generally rangedfrom 40-150 nm but can also be prepared to have diameters less than 40nm and sometimes less than 20 nm. The integrity of the 3-D solid CB×NTmaterial was maintained due to the boron induced “elbow”defects—promoting tube entanglement and nanojunctions during synthesis.These defects are shown in FIGS. 1C-1F. FIG. 1C is an ion beam slice andview SEM image showing the interior of the CB×NT material includingpurely CB×NTs with no trace of amorphous carbon. FIG. 1D is a closerlook at the “elbow” defects found in the CB×NT material. FIG. 1E is anSTEM image showing two, four-way covalent nanojunctions in series of theCB×NT material. FIG. 1F is a TEM image showing two overlapping CB×NT'swelded together assisted by boron doping.

Raman Spectra

Raman Spectra was used to compare pristine carbon nanotubes with theCB×NT material. The Raman spectroscopy was done using a Renishaw systemwith laser excitation line λ=514 nm and λ=633 nm. FIGS. 4A-4B are graphsthat show the Raman spectroscopy comparison of pristine carbon nanotubeswith CB×NT material using the 514 nm and 633 nm wavelength lasers,respectively. These figures evidence that there is a strong D-modeintensity compared to the G-mode intensity, which is believed to be dueto the “elbow” defect morphology induced by the boron doping in thehexagonal carbon network.

An intense disorder peak (D-band) (˜1300-1360 cm⁻¹) compared to theG-peak intensity (˜1590 cm⁻¹) is seen in the CB×NT material, which wouldbe expected considering the contribution of these substitutional defectsinduced by the existence of boron in the hexagonal sp² hybridizednetwork of the CB×NT material. These elbows may be explained by assumingthe boron is substitutionally replacing carbon atoms in these regions,favoring the pentagon heptagon pairs to create stable bends (positiveand negative curvature) as predicted by theory. [Sumpter 2009]

Boron induces atomic-scale “elbow” junctions, as depicted in FIG. 1D andFIGS. 5A-5H, and many other fascinating nanotube morphologies includingcovalent multi-junctions, such as Y-junctions (FIGS. 6A-6F), andfour-way junctions (FIGS. 1E-1F). The most abundant morphologies werethe stable “elbow” bends (exhibiting positive and negative curvature)which were found to be continuous and somewhat periodic along the tubelength. FIGS. 6A-6F. The porosity could be obtained by recording the N₂gas absorption isotherm using the Brunauer-Emmett-Teller (BET) analysistechnique. After performing this test, the results show a type-IIadsorption isotherm (FIG. 9A), exhibiting a negligible concave section,which was attributed to microporous volume uptake, and a rapid rise intotal volume near P/P₀=1; a macroporous material (pore diameters>50 nm).If it is assumed that the density of individual MWCNTs to be around 2.1g/cm [Stephan 1994; Lehman 2011], any sample with a density<19 mg/cm³would have a porosity>99% (thus meaning that 99% of the volume is air).The BET surface area measurement further characterizes the material aswell. The BET surface area was found to between 103.24 m²/g and 360.42m²/g. FIGS. 9B-9C.

EELS Mapping of Boron

From these microscopy studies, it was observed that the CB×NT materialwas entirely made up of CB×NT with little to no amorphous carbon, andseveral nanojunctions and branches were observed which may also have arole to the structural integrity of the CB×NT materials.

A dramatic increase in the population of elbowed morphologies associatedwas observed with increasing boron content. The location of boron withinthe carbon nanotubes was mapped using high-angle annular-dark-field(HAADF) imaging (HAADF) and EELS linescans using a 0.7 nm STEM probe.

The mechanism driving these stable “elbow” formations can be explainedby the high stability of boron atoms (or other heteroatoms) on negativeGaussian curvature sites, thus present in rings with more than sixcarbon atoms (heptagons or octagons). To confirm the effect of boron onnegatively curved sites, first principles calculations based on theDensity Functional Theory (DFT) were carried out (using a plane wavebasis code (VASP) under the GGA/PBE approximation [Kresse 1996; Perdew1996] to simulate doped “elbow” shape nanostructures. See boomerang-typetube in FIG. 8. A plane-wave basis with a 400 eV energy cut-off wasemployed, and each structure was relaxed down to 0.001 eV/A for eachdopant position. According to these calculations, the substitutionalenergy, for boron doping the boomerang-type structure, is the lowest atheptagonal rings (negative Gaussian curvature; K<0), whereas nitrogenatoms are favored at the pentagonal sites (positive curvature whichcauses closure of the structure; K>0). However, sulfur can beaccommodated at both, heptagons and pentagons, thus promoting branchingof multi-walled carbon nanotubes [Romo-Herrera 2009; Romo-Herrera 2008].It is therefore clear that the selective preference for negativeGaussian curvature (K<0) of boron, and its influence in inhibiting theformation of pentagons that avoid tube closure (e.g. continuous growthof opened tubes), make boron a dopant able to catalyze the growth ofthese long, entangled and novel CB×NT materials.

The location of boron within the CNTs was experimentally mapped usinghigh-angle annular-dark-field (HAADF) imaging and electron energy lossspectroscopy (EELS) line-scans using a 0.7 nm STEM probe. Line-scanswere recorded along the edges of the tube in the region of the “elbow”defects. FIGS. 9A-9G. The regions of highest boron (B) concentrationwere found to be at the location of the “elbows,” supporting that boronplayed a key role in the formation of negative curvature areas inducingthe formation of “elbow” junctions. As shown in FIGS. 9A-9B, linescansacross the tube diameter failed to reveal the presence of boron withinthe inner tube or catalyst particles, thus confirming that the B atomswere incorporated mostly within the walls of the CNTs. As shown in FIGS.9C-9F, this trend was also observed in other line scans. In FIG. 9G, theEELS survey spectrum, including the characteristic B K-shell peaks and CK-shell peaks (in curve 912), further confirmed the presence of B atomsbeing at concentrations well within the EELS detection limit.

Additional EELS linescans made along localized regions of high positiveand negative curvature (FIGS. 10A-10G) both show boron signals ascompared to the artifact measurements from the “vacuum region.” As shownin FIG. 9G, the C K-edge shows maximum peaks at 287.2 and 295.4 eV whichcorrespond to the 1s π* and 1s σ* resonance respectively. Meanwhile, theB K-edge shows maximum peaks at 193.2 and 202 eV corresponding to the 1sπ* and 1s σ* resonance respectively. The 1s π* resonance is indicativeof sp² hybridization, which indicates that boron is bonded to carbonwithin the carbon nanotube lattice.

XPS

Furthermore, it is well known that boron-doped CNTs enhance theiroxidation resistance. [Perdew 1996; Yang 2011]. For this reason, theEELS elemental survey data (FIG. 9D) shows higher energy boron bondingstates to be dominant, thus indicating the accumulation of the glassyboron oxide layer coating the CNTs surface. [Yang 2011]³⁰ The XPspectrum revealed five underlying B1s bonding states located at peakpositions 187.6, 188.8, 190.1, 191.9, and 193.4 eV as marked as peaks inFIG. 11. The lower energy peaks are associated with the B—C bonding,such as that in B₄C (187.8 eV), and B-substituted-C (188.8 eV) withinthe graphite crystal structure as proposed by Cermignani 1995 andreported by others [Wu 2005; Liu 2009; Jacobsohn 2004; Shirasaki 2000;Burgess 2008]. The higher energy components represent the more oxidizedspecies corresponding to BC₂O (190.0 eV), BCO₂ (192.0 eV), and B₂O₃(193.2 eV) respectively. It was found that the main bonding state ofboron in the sponges, consist of sp² hybridization of B-substituted-C,which supports theoretical calculations. Quantitative elemental analysisof the boron content in this structure was revealed to be ca. 0.7 at %.

For the XPS characterization, the chemical bonding states and atomicquantification of boron content within the CB×NT solids were studiedusing a Phi Quantera instrument equipped with monochromatic A1 (K−α)1486.6 eV X-ray source at 50 W and a 200-μm-beam diameter. Argonion-etch pre-treatment was performed for 2 minutes using a 3 kV beam andtarget emission current at 7 mA. Survey scans were performed at 140 eVpass energy, and C1s and B1s elemental scans at 55 eV pass energy with0.10 eV steps. Data analysis software was used for the peak fittingusing Guassian functions and a linear baseline. All peaks were generatedhaving FWHM limited to <2.0 eV. Our fits provided values ofchi-squared=8.127 and R²=0.978. Before fitting, the background wassubtracted using the software and the peaks were calibrated to the C1speak located at 184.6 eV for graphitic carbon.

Dynamic Mechanical Analysis (DMA)

In addition to the formation of the 3D structure, the CB×NT materialsexhibited robust flexibility (FIGS. 1A-1B and SA-SE) and good isotropicelastic mechanical behavior. Dynamic mechanical tests were performed onCB×NT material samples using an Intstron Electropuls E3000 instrumentand Wavematrix software. As shown in FIG. 12A, at 60% compressive strainthe complex modulus is E*=1.26 MPa and tan δ=0.058. The high tan δ value(ratio of the loss modulus E″, to storage modulus E′) was indicative ofthe materials high energy absorption and rubber-like damping capability.Due to the randomly orientated entangled CNT network, isotropic behaviorwas not surprising as compared to conventional anisotropic MWCNT alignedarrays as depicted in the model drawing of FIG. 12B. Only 18% plasticstrain deformation was observed after dynamic compression of 100 cyclesat 60% strain. Further dynamic mechanical analysis (DMA) gave tan δ≈0.11at ˜9% strain (FIG. 12C) in a multi-strain (ramp strain) test mode. At adynamic strain of 10% over 11,000 cycles (FIG. 12D), a gradual increasein damping was observed, from tan δ≈0.11 to tan δ≈0.13, along withincreasing stiffness and stress levels.

For the lower strain amplitude tests at ˜9% strain, TA instruments DMAmodel Q800 was used. Tan δ values (the ratio of loss modulus, E″, tostorage modulus, E′) and sample stiffness data were measured from asponge block with density≈25 mg/cm³ and size: 2.4015 mm×11.2167 mm²under compression tests in multi-strain mode with amplitudes ranging0-300 μm (corresponding to strains up to ˜9%) at a frequency of 1 Hz and0.01 N preload force. A total of 50 data points were collected from 250cycles to make the plot of FIG. 12C, with each sequential cycle and datapoint having a linear increase in strain amplitude from 0-300 μm(corresponding to strains of 0-9%).

DMA was limited to stay within the perfectly elastic regime of the CB×NTmaterial, limiting the analysis to minimal strain levels. It was noticedthat the higher density samples resulted in higher stress levels, asexpected, being that denser samples have more network elements in thestructure. Each “elbow” joint within the CB×NT materials may act as a‘spring’ joint to provide reversible elastic deformation. See FIG. 12A.Strain amplitude on the DMA instrument was limited to the perfectlyelastic regime of the material, therefore, above 10% strain, sampleswould start to plastically deform and the instrument would end the testas to confirm full strain recovery during testing. As a comparison,viscoelastic polymers have E*˜20 MPa and tan δ≈1.0 while most hardplastics have E*˜1 to 10 GPa and tan δ range from 0.01 to 0.10. In astiffness-loss map, the CB×NT viscoelastic solids may be categorizedwith rubber foam. Although the mechanical data suggested the degree ofcovalent bonding in these solids to be fairly low, it was enough toyield perfectly self-intact elastic solids (after only 30 minutes ofgrowth) up to 10% strains before seeing any loss in volume; and only 18%plastic strain deformation after 100 cycles at 60% strain. Theincreasing stiffness with sequential compression cycles may indicatesome CNT alignment in the structure along the compression axis. FIG.12D.

It was noticed that the higher density samples resulted in higher stresslevels as expected, and the samples were mechanically isotropic due toits random entangled 3D network, similar to the findings of “CNTsponges” recently reported. [Gui I 2010; Gui II 2010]. Control of thedensity and overall stiffness (resilience) of the sponges can be carriedout by changing the solution feed rate. It has been found that a lowerfeed rate yielded lower densities and more resilient and more flexiblesponge-like material.

Exploiting the super-hydrophobic nature of CNTs [Li 2002] and thelow-density 3D porous framework, the sponge-like solid was shown to beusable as a reusable oil sorbent material in seawater. As shown in FIGS.13A-13B, the CB×NT material had a contact angle around 150° with a 2 mmwater droplet, but readily absorbed many organic compounds andhydrocarbons, including alcohols and oils.

Strong oleophilic behavior was observed with very high absorptioncapacity. Weight-to-weight absorption capacity (defined by W (g g−1),the ratio of the final weight after absorption and the initial weightbefore absorption) for common solvents was measured on CB×NT spongeswith three different densities: 24.3 mg/cm³, 17.3 mg/cm³, and 10.8mg/cm³, and plotted as Lines 1301-1303, respectfully in FIG. 13C. Theabsorption capacity values, W (g g−1), were obtained by measuring themass of the dry as-produced sponge, and then measuring the mass afteroil/solvent absorption. The ratio of the final mass to the initial masswas taken as the W (g g−1) value, averaging out three samples. To ensurefull saturation was obtained before weighing, the samples were leftsubmerged in the solvent/oil (without water) overnight. The samples werethen removed with sharp needle tweezers and immediately placed onto aweigh paper to be measured on the mass balance.

Table I reflects the solvent weight-to-weight absorption data for theCB×NT sponges for each of the three different densities of 24.3 mg/cm³,17.3 mg/cm³, and 10.8 mg/cm³.

TABLE I Sponge Sponge Sponge ρ = ρ = ρ = Solvent 24.3 mg/cc 17.3 mg/cc10.8 mg/cc Hexanes (0.6548 g/ml) 26.00 29.61 44.37 Ethanol (0.789 g/ml)30.65 33.14 62.61 Kerosene (0.81 g/ml) 31.99 36.81 59.29 Toluene (0.867g/ml) 37.38 48.46 65.48 Used Engine Oil (0.913 g/ml) 41.06 54.45 78.85Ethylene Glycol (1.1132 g/ml) 52.98 74.38 79.526 Chloroform (1.483 g/ml)62.28 76.91 122.86

As shown in Table 1, increasing solvent density and decreasing CB×NTsponge density resulted in higher absorption capacity. W increased withlower density sponges and with higher density solvents with as high asW=123 for chloroform (1.483 g/cm³) and as low as W=22 for hexanes (0.655g/cm³).

The volume-to-volume absorption capacity (defined by V, the volume ofthe solvent absorbed by the CB×NT sponge per unit volume of the CB×NTsponge before absorption) was calculated from this same data. Table IIreflects the volume of solvent absorbed per unit volume of the CB×NTsponges for each of the three different densities of 24.3 mg/cm³, 17.3mg/cm³, and 10.8 mg/cm³.

TABLE II Sponge Sponge Sponge ρ = ρ = ρ = Solvent 24.3 mg/cc 17.3 mg/cc10.8 mg/cc Hexanes (0.6548 g/ml) 92.78% 75.59% 71.53% Ethanol (0.789g/ml) 91.32% 70.47% 84.33% Kerosene (0.81 g/ml) 92.97% 76.48% 77.72%Toluene (0.867 g/ml) 101.96% 94.70% 80.32% Used Engine Oil (0.913 g/ml)106.62% 101.28% 92.09% Ethylene Glycol (1.1132 g/ml) 113.47% 114.04%76.18% Chloroform (1.483 g/ml) 100.41% 88.55% 88.74%

As shown in Table II, the volume of solvent the CB×NT sponges absorbedwas between about 70% and about 115% of the volume of the CB×NT spongebefore absorption.

The combination of high electrical conductivity with low density andporosity is another interesting aspect of this CB×NT material. As shownin FIG. 7B, the surface area of the CB×NT material with density≈25mg/cm³ was determined to be 103.24 m²/g by BET analysis. BET surfacearea analysis was done using Quantachrome Autosorb-3B Surface Analyzerand Autosorb 6 software. As shown by curve 1404 in FIG. 14B, the bulkelastic solids had an electrical resistivity around 2.4×10⁻²Ω-m at atemperature of 2 K and decreased to 2.0×10⁻²Ω-m at 395 K via four probemeasurements. Four-probe electrical resistivity and magnetic momentmeasurements were done using a physical properties measurement system(PPMS; Quantum Design).

As noted above, experimental parameters can be varied (tailored) forcreating a structure of desired properties such as density, porosity,surface area, carbon nanotube diameter, boron doping concentration,etc., and boron content. Experimental parameters might change to someextent for optimizing and controlling growth on a new system. Changingsynthesis parameters such as dopant concentration and temperature, givethe ability to control the boron defect concentration, density ofjunctions, and the overall properties of the CB×NT materials.Furthermore, these defects could act as anchor points for chemical orcluster functionalization in order to better tailor CB×NT for variousalternative applications.

Varying the synthesis growth time will enhance the structural andmechanical integrity of the entangled network as longer carbon nanotubeswill make the CB×NT materials less brittle and less likely to crumble.The metal catalyst (iron, nickel, cobalt, etc.) can also be changed.Carrier gas composition, gas flow rates, solution feed rates, density,porosity, boron concentrations (elbow, defect concentrations), nanotubediameters, number of nanotube walls may also be varied. Compositematerial variations can be realized. This includes chemicalfunctionalizing, which will affect the properties of the CB×NT materialsand physadsorbing metal nanoparticles to the surface of the CB×NT fortailoring selective adsorption of chemical species etc.

Functionalization of the Heteroatom Doped Carbon Nanotube Material

In embodiments of the present invention, the heteroatom doped carbonnanotube materials (such as CB×NT materials) can be functionalizationwith metal nanoparticles (such as Au, Pt, Ag, Ti, Ni, Sc, etc.) or withother chemical receptors, polymers, proteins, etc. For instance, forCB×NT materials, the heavily boron-doped regions may act as chemicalanchor points for functionalization or adsorption of specific gasmolecules or solid-state particles, such as metals or metal ions forexample.

The CB×NT materials (or other heteroatom-doped carbon nanotubematerials) can also be functionalized using processes similar to thosefor functionalizing carbon nanotubes, such as, for example, by processessimilar to those disclosed and taught in Margrave '455 Patent, Colbert'098 Patent, Khabashesku '533 Patent, Tour '147 Patent, and Tour '137Patent.

Polymer Composites of the Heteroatom Doped Carbon Nanotube Material

In embodiments of the present invention, the heteroatom-doped carbonnanotube materials (such as CB×NT materials) can used to form acomposite with a polymer binder.

For instance, the CB×NTs (or other heteroatom-doped carbon nanotubes)can be functionalized (such as described above) and then a polymer canbe bound (by polymerization or otherwise) to the CB×NTs, such as, forexample, by using processes similar to those disclosed and taught inTour '940 Patent, Tour '137 Patent, and Tour '103.

Further, for instance, a polymer can be directly bound to the CB×NT,such as, for example, using a process similar to those disclosed andtaught in Tour '199 Patent.

Also, for instance, a polymer matrix can be used to bind the CB×NTmaterial, such as, for example, using a process similar to thosedisclosed and taught in Smalley '596 Patent.

Use of the Heteroatom-Doped Carbon Nanotube Material in Processes

By the present invention, it has been discovered that doping of carbonnanotubes with heteroatoms (such as elemental boron) created an entirelydifferent tubule morphology, “elbow” geometrical defect, in the carbonnanotube lattice giving it unique material properties including:chemical, physical, mechanical, and electrical (altered thermal andoptical properties are yet to be discovered). The synthesis parametersstated above produced high yields of a 3-dimensional, low density,porous solid sponge-like material composed of a heavily entanglednetwork entirely of clean (little to no amorphous carbon) CB×NTs, whichare generally boron-doped multiwall carbon nanotubes (CB×NT's). Thisnanostructure of the CB×NT material remained self-intact upon manydeformation cycles without the need of any polymer binding material(s)to form a composite. In this way, the exceptional electricalconductivity that nanotubes have to offer was not compromised, which maybe useful for some applications requiring such high conductivity. Infact, the boron doping seemed to alter the electronic properties of thecarbon nanotubes making it an even more conductive bulk material thanits pristine counterpart.

This synthesis procedure of the present invention takes advantage of thefact that boron acts as a “surface-active agent” during growth of carbonnanotubes producing higher yields than its pristine carbon nanotubecounterparts (and even higher than nitrogen doping for that matter,which actually has been proven to slow down growth rate). Therefore,novel and unique aspects of the present invention include:

This synthesis procedure has shown to be feasible at the large-scaleindustrial level; considering the low cost of production and the factthat the yields are so high (˜66-100 mg/min).

The heavily boron-doped regions may act as chemical anchor points forfunctionalization or adsorption of specific gas molecules or solid-stateparticles, such as metals or metal ions for example.

There is a dramatic photo-conductivity in tandem with strongphoto-acoustic and photoelectric effect exhibited upon exposure to anintense flash of light (i.e., a camera flash). This effect was observedto be more dramatic than that of its pristine carbon nanotubecounterpart, producing a ‘popping’ sound and emitted sparks in response.

These and other novel and unique aspects of the present invention can berealized in many applications for which this material can be utilized.High-tech and high quality sponges are realized with extremely lowdensity, robust mechanical properties, high porosity, superhydrophobicity and high specific surface area while maintaining veryhigh electrical conductivity. These are a remarkable combination ofmaterial properties that signifies the novelty and uniqueness of thismaterial.

The present invention can be utilized in a number of applications,including:

Environmental

Embodiments of the present invention can be used for cleaning oilspills, for waste water purification (such as desalination ordeionization), etc. Highly efficient natural and synthetic sorbentmaterials are of current interest for environmental applications on aglobal scale regarding the increased risk in oil spill catastrophe. Inthis regard, heteroatom-doped carbon nanotube materials (or “sponges”)are superior when it comes to mass absorption efficiency.

As discussed above, FIG. 13C is a graph of the weight-to-weightabsorption capacity defined by the ratio of (a) the final weight aftersolvent absorption to (b) the initial weight of the sponge beforeabsorption for common solvents, as measured on samples having differentdensities. Lines 1301-1303 are for samples having densities of 24.3mg/cm³, 17.3 mg/cm³, and 10.8 mg/cm³, respectively. This absorptioncapacity of the CB×NT material renders it capable for use as a sorbentmaterial.

The use of this sorbent material to clean environmental oil spills wastested using seawater (Galveston, Tex., USA) and black engine oilretrieved from a Houston, Tex., USA gas station (ρ≈0.913 g/cm³). Theresults of this testing reveals weight-to-weight absorption capacity ashigh as 79 (i.e., W=79) with the 10.8 mg/cm³ samples. By way ofcomparison, woolspill knops, the leading natural oil sorbent material(having a density as low as 33 mg/cm³) has a weight-to-weight absorptioncapacity of 36 (i.e., W=36) with heavy fuel oil (ρ≈0.9535 g/cm³). [SeeMcFarland '215 Patent]. Woolspill™ knops, like most natural sorbents,are hydrophilic and water uptake is expected to minimize its efficiency;therefore the super-hydrophobicity of CNT sorbent materials is a clearadvantage.

In this testing, the high buoyancy in seawater was demonstrated byforcing the sample underwater and observing the speed at which thesample submerged to the surface remaining completely dry.

Based on the oil absorption property, a sequence of events wereperformed to demonstrate the materials use to clean environmental oilcontamination in seawater. FIGS. 13D-13G. After the CB×NT material 1304becomes saturated, the oil could be burned out, and the material couldbe reused in this way time and time again. The burning of theoil/solvent saturated material will not destroy the CB×NTs significantlysince the more volatile substance (oil/solvent) takes most of the localoxygen meanwhile it coats (protects) the CNTs' surface from oxidation.As the solvent/oil vanishes, the CNTs would then have less protectionagainst oxidation, and the CNTs would only then begin to burn to a smalldegree before it rapidly cools below the oxidation temperature.Alternatively, the oil can be salvaged by means of squeezing it out bymechanical compression (inset 1307 of FIG. 13F). The CB×NT material canbe used to ‘mop-up’ the surface oil out of the water as demonstrated byusing a permanent magnet (field strength ˜2000 Oe) to move it around asa means to tracking the oil spills. The ferromagnetic properties of thesponge arise due to the iron catalyst particles used in the growthprocess that remains trapped in the CNT core.

As shown in FIG. 14A, the room temperature magnetization curve 1401(M(H)) indicates a very high coercive field of iron (Fe) catalystnanoparticles (˜400 Oe) when compared to bulk iron (˜0.9 Oe). Curve 1403in inset 1402 reflects the coercivity of the sample was found to be ˜400Oe. The strain induced anisotropy due to the spatial confinement withinthe nanotube may be the reason for the high coercivity found in thesemagnetic CB×NTs. The considerable saturation magnetization and low fieldof saturation enables magnetic tracking even with small magnetic fields.

Interestingly, this heteroatom-doped carbon nanotube materials, such asCB×NT materials, possess a combination of physical properties that willimpact the practical use of CNTs for this application. The result ofmacroscopic CB×NT materials having a network containing many covalentinterconnections makes this application more feasible, and helps todeter the drawback of environmental impact concerns of nanoscale debris.Having the ability to direct its whereabouts (oil tracking) via magneticfield further offers a controllable way for handling and recovering allCNT material more safely.

Energy Application

Being a porous and highly conductive bulk material, embodiments of thepresent invention can be used as an electrode material forsupercapacitors/battery devices. Three-dimensional Li ion batterydevices are envisaged by coating the heteroatom doped CNTs with anelectrically insulating but Li ion conducting layer (which may beaccomplished by a low viscosity liquid solution dip coating procedure)followed by filling the remaining free volume of the macropores withcathode material. This would operate as a flexible 3D battery device.

Energy Scavenging/Harvesting Technology

Nanopiezotronic materials may be constructed by making composites withthe 3D CNT macrostructure as a conductive scaffold support withinorganic piezoelectric materials such as zinc oxide (ZnO),nanotubes/nanorods or piezoelectric polymers such as polyvinylidenefluoride (PVDF). The 3D CNT macroscale framework may be used as atemplate/substrate for ZnO nanowire growth/synthesis either in theliquid phase or vapor phase via CVD process procedures. PVDF can beinfiltrated and electrostatically poled with high electrical field overset number of minutes/hours to align the polymer chains for maximumpiezo-response. Microwave welding procedures may be utilized to weldpolymers to the CNT framework for better anchoring.

Bioengineering Material

Embodiments of the present invention can be used as tissue engineeringscaffold support for cell growth such as skin/tissue/muscle/bone growth,etc. The high electrical conductivity of the 3D CNT macroscale scaffoldmay be exploited in this application to stimulate artificial muscleactuation by electrical impulses. In this regard composites thereof maybe considered with electrolyte filled polymers and polymer solutionsthereof to create electrochemical charge injection type stimulation foractuation.

Hazardous Gas Sensor Application

Embodiments of the present invention can be used to detect harmfulgases/chemical vapors in the ppm range, such as by a detectable changein electrical conductivity due to physisorption.

Mechanical Sensor Applications

Embodiments of the present invention may be used to detect super lowstrains, such as by a detectable change in the electrical conductivity.There may be novel electromechanical phenomenon at the nanoscale “elbow”joints found in the heteroatom-doped macroscale solids. For example, theregions of high curvature may be active sites for electromechanicalcoupling phenomena upon induced mechanical strains (regions of highstress concentration) originating from quantum mechanical manifestationsat the atomic level [ref. Taganstev et. al. MRS Bulletin Vol. 34 2009].

Hydrogen Storage

Embodiments of the present invention can be used for hydrogen storage,which may have applications in automobiles. The 3D CNT macrostructuremay be decorated with any metal or transition metal nanoparticles (Ag,Au, Pt, Pd, Ni, Ti, Sc, V, Cu, etc.), or metal hydride nanoclusters,which may enhance the binding affinity for H₂ absorption anddissociation.

Other Gas Storage/Capture Applications

H₂S₂, HS₂, CO₂, CO, NH₃, NO₂, etc. gas capture/storage may be usefultowards environmental needs. In which the 3D CNT macrostructures may befunctionalized or decorated with nanoparticles for tailoring selectivityand enhancing the molecular gas' binding energy to the CNT surface andfilling the pore volume.

Nuclear Applications

Boron carbide (B₄C) is known to have good neutron absorption efficiencywithout forming long-lived radionuclides, which makes it attractive asan absorbent for neutron radiation arising in nuclear power plants. Suchapplications include shielding materials or control rod materials. Theboron content or the B₄C within the 3D MWCNT macrostructure may beenhanced using the microwave irradiation energy mixed with a boronpowder source within the 3D MWCNT macrostructure material.

Other Applications

Embodiments of the present invention can be used for other applications,such as material applications requiring a superhydrophobic surfaceand/or oleophilic surfaces.

The mechanically robustness of embodiments of the present invention maybe changed or altered in view of the desired application. For instance,polymers may be used to form a composite as discussed above. In someembodiments, the material may be self-sufficient for the desiredapplication.

The examples provided herein are to more fully illustrate some of theembodiments of the present invention. It should be appreciated by thoseof skill in the art that the techniques disclosed in the examples whichfollow represent techniques discovered by the inventors to function wellin the practice of the invention, and thus can be considered toconstitute exemplary modes for its practice. However, those of skill inthe art should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments that are disclosed andstill obtain a like or similar result without departing from the spiritand scope of the invention.

REFERENCES

Further references in the field of the present invention include:

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All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. A method for making solid three-dimensionalcarbon nanotube structures comprising boron-containing carbon nanotubesdirectly during chemical vapor deposition synthesis of carbon nanotubes,comprising reacting a hydrocarbon, a boron source, and a metal catalystsource into a chemical vapor deposition reactor, wherein the ratio ofthe metal atoms to the boron atoms is between 2 and 20, wherein thesolid three-dimensional carbon nanotube structures comprisingboron-containing carbon nanotubes form a macroscale three-dimensionalmaterial of entangled carbon nanotube networks.
 2. The method of claim1, wherein the ratio of the metal atoms to the boron atoms present isbetween 4 and
 6. 3. The method of claim 1, wherein the carbon sourcecomprises toluene or hexane.
 4. The method of claim 1, wherein the metalcatalyst source comprises ferrocene.
 5. The method of claim 1, whereinthe boron source comprises triethylborane.
 6. The method of claim 1,wherein the hydrocarbon, the boron source, and the metal catalyst sourceare combined to form a chemical precursor within the reactor.
 7. Themethod of claim 6, wherein the hydrocarbon is between about 87 wt % andabout 97 wt % of the hydrocarbon, the metal catalyst source, and theboron source.
 8. The method of claim 6, wherein the metal catalystsource is between about 2.5 wt % and about 12 wt % of the hydrocarbon,the metal catalyst source, and the boron source.
 9. The method of claim6, wherein the boron source is between about 0.1 wt % and about 2 wt %of the hydrocarbon, the metal catalyst source, and the boron source. 10.The method of claim 1, wherein the three-dimensional carbon nanotubestructures comprising boron-containing carbon nanotubes form amacroscale three-dimensional structure of nanotubes wherein themacrostructure is at least 1 cm in two perpendicular directions.
 11. Themethod of claim 1, wherein the three-dimensional carbon nanotubestructures comprising boron-containing carbon nanotubes form amacroscale three-dimensional structure of nanotubes wherein themacrostructure is at least 1 cm in three orthogonal directions.
 12. Themethod of claim 1, wherein the three-dimensional carbon nanotubestructures comprising boron-containing carbon nanotubes are porousmaterials having a bulk density less than 10 mg/cm³.
 13. The method ofclaim 1, wherein the three-dimensional carbon nanotube structurescomprising boron-containing carbon nanotubes are porous materials havinga bulk density between 10 mg/cm³ and 29 mg/cm³.
 14. The method of claim1, wherein the three-dimensional carbon nanotube structures comprisingboron-containing carbon nanotubes are comprised of an isotropic ensembleof entangled individual carbon nanotubes comprising elbow defects orcovalent junction sites.
 15. The method of claim 1, wherein thethree-dimensional carbon nanotube structures comprising boron-containingcarbon nanotubes are comprised of an isotropic ensemble of entangledindividual carbon nanotubes which does not comprise elbow defects orcovalent junction sites.
 16. The method of claim 1, wherein the boronsource is selected from the group consisting of organoboranes,organoborates, and combinations thereof.
 17. The method of claim 1,wherein the three-dimensional carbon nanotube structures comprisingboron-containing carbon nanotubes exhibit elastic mechanical behavior.18. The method of claim 1, wherein the three-dimensional carbon nanotubestructures comprising boron-containing carbon nanotubes exhibit magneticproperties with a high coercive field of about 400 Oersted.
 19. Themethod of claim 1, wherein the three-dimensional carbon nanotubestructures comprising boron-containing carbon nanotubes are poroussolids.
 20. The method of claim 1, further comprising welding theboron-containing carbon nanotubes of the three-dimensional macroscalecarbon nanotube structures.
 21. The method of claim 1, wherein the boronsource is the boron containing gas source, boron trichloride.